The present invention relates to apparatus for improving the linearity of a phase or frequency modulated oscillator, operating preferably within the microwave range. The apparatus according to the invention is applicable to oscillators which are used in broad band radio links, for example, for television transmission having great demands on noise characteristics, linearity and transmission capacity.
There are known frequency modulated microwave oscillators which utilize for frequency modulation active semi-conductor elements, for example, avalanche or Gunn diodes and capacitance diodes (varactor diodes). The U.S. Pat. No. 3,675,161, for example, describes a varactor controlled microwave semi-conductor oscillator in which an electromagnetic wave of microwave frequency is generated by means of a reversed biased semi-conductor diode of the avalanche type disposed in a cavity resonator. A varactor diode is used as a variable reactance element coupled to the semi-conductor diode for controlling the frequency of the generated wave. The semi-conductor diode acting as a negative resistance is connected to an external voltage, while the varactor diode is connected to a variable voltage source for varying the capacitance of the diode so that the frequency of the wave generated by the oscillator is changed. In this manner a microwave field is obtained from the oscillator cavity which is frequency modulated by the modulated signal and which is superimposed on the voltage supplied to the varactor diode.
One problem with frequency modulated oscillators of the above mentioned kind is that the relation between the frequency of the microwave signal generated in the oscillator and the modulating voltage is not linear. The reason for this non-linearity originates mainly from the non-linear properties of the varactor diode operating as frequency modulator. Furthermore, the generated microwave field contains harmonics of the second and higher orders due to the non-linearity in the frequency determining semi-conductor diode. However, it has been discovered that the presence of at least the first harmonic in the generated microwave field has a favorable influence on the efficiency of a frequency modulated oscillator which contains a Gunn diode or an avalanche diode as an active element.
A number of methods have previously been suggested to improve the linearity of a single tuned varactor modulated microwave oscillator. One method is to divide the original resonator to form two coupled resonators, one containing the varactor diode and the other containing the active semi-conductor element, for example, a Gunn diode. The latter resonator should then have the same construction as the original. By variation of the resonator parameters as, for example, the Q-value and the coupling factor between the resonators an improved linearity of the modulator can be obtained. The drawback with this known arrangement is mainly that a greater power loss is obtained and that the separate varactor resonator which is tuned closely to the oscillator frequency unfavorably affects the tuning of the frequency determining resonator and affects the matching to the load.
From, for example, U.S. Pat. No. 3,435,374, furthermore, it is known to improve the tuning properties of an oscillator containing a non-linear negative resistance by coupling one or several tuned resonant circuits to the included active element.
An object of the present invention is to provide apparatus for improving the linearity in a frequency or a phase modulated oscillator, preferably operating within the microwave range, wherein the apparatus at least partly compensates the influence of the first order distortion in the obtained modulated signal, whereby improved linearity of the oscillator is obtained with negligble power loss.